A steadily increasing demand for portable electric power has stimulated interest in the development of more efficient and more powerful fuel cell devices. A polymer electrolyte membrane (PEM) fuel cell is a strong candidate as a portable power source for commercial applications primarily because of its low weight and high power density.
The operation of a PEM fuel cell relies upon the proton conductivity properties of a polymeric membrane positioned between the two electrodes of the cell, to transport protons internally from one electrode to the other. The membrane must also have no electronic conductivity, good chemical and mechanical stability, and sufficient gas impermeability to prevent cross over of the fuel. For many years now, the membrane of choice has been a sulfonated perfluoro polymer known as Nafion®, commercially available from DuPont. Nafion is a copolymer of tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octenesulfonyl fluoride, available in acid or ionomer form.
The major drawback to Nafion as the ideal polymer electrolyte membrane in fuel cells is that its proton conductivity depends on the water content in the membrane, in which proton transport is based on the diffusion of hydronium ion (H3O+). In order to retain its high proton conductivity, Nafion membrane requires the use of pre-humidified gases at an operating temperature under 80° C. Such requirements considerably increase the cost, size and complexity of PEM fuel cells using Nafion. Nafion membranes cannot perform under dry or low relative humidity conditions nor above the boiling point of water, despite the faster chemical reaction and increased output that would result from the higher temperature. Furthermore, operating at the lower temperature required by Nafion increases the risk of carbon monoxide poisoning of the fuel cell catalyst.
Various attempts have been made to develop water-free proton conductive membranes for PEM fuel cells that do not have the low temperature and high relative humidity requirements of Nafion. One such attempt, for example, is described in the Hinokuma et al. U.S. Pat. No. 6,495,290, issued Dec. 17, 2002, incorporated herein by reference. The proton conductors employed by Hinokuma et al. are based on fullerene derivatives containing acidic functional groups such as —OH or —SO3H, and are designed to operate under dry conditions over a wide range of temperatures. The proton conductors are described as being either compacted powder of the fullerene derivatives, or mixed with a small amount, generally 20 weight percent or less, of a film-forming polymeric material, such as polytetrafluoroethylene, polyvinylidene fluoride or polyvinyl alcohol. The patent cautions against employing the polymer in amounts any greater than 20 weight percent, at the risk of degrading the proton conductivity of the fullerene derivative. Furthermore, there is no hint in the Hinokuma et al. patent of using the fullerene derivative in combination with Nafion.